1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Wireless communication systems typically include base stations that provide wireless connectivity to cover a geographical area such as a cell or a sector of a cell. The base stations communicate with mobile units in the cell or sector over an air interface. The air interface supports downlink (or forward link) communication from the base station to the mobile unit and uplink (or reverse link) communication from the mobile unit to the base station. The uplink and downlink communication uses corresponding uplink and downlink channels, which may be realized by use of carrier frequency, modulation, coding, frequency/time multiplexing, multiple antenna techniques, or combination thereof. Examples of standards and protocols that are used to define uplink and/or downlink channels include Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), Multiple Input Multiple Output (MIMO), Code Division Multiple Access (CDMA), Orthogonal Frequency Division Multiple Access (OFDMA), Spatial Division Multiple Access (SDMA), Single Carrier Frequency Division Multiple Access (SC-FDMA), and the like.
An antenna array can be used to steer a beam toward a target mobile in the forward link and to control the coverage area of a sector served by a base station. For example, a phased-array system comprised of closely spaced antennas can be used to control the beamwidth of the antenna array (e.g., beamforming) and to control the pointing direction of the antenna main lobe (e.g., beamsteering). A base station can use an antenna array to steer a beam for forward link traffic signals to a target mobile in the sector, thereby reducing transmission power and hence overall co-channel interference level in its sector and adjacent sectors. While beamsteering is used to direct individual narrow beams containing traffic information to different mobiles, common broadcast channels from the base station are received by all mobiles in the sector simultaneously and must have a beamwidth broader than individual traffic beams to cover the whole sector coverage. The beam for the common broadcast channels is fixed and is pointed toward the central line of the sector.
The beamwidth of common broadcast channels transmitted by an antenna array is usually narrower than the beamwidth of any single one of the antennas in the array. For example, in an antenna array with half-wavelength antenna spacing, the beamwidth of a vertical polarization antenna is about 110° while the beamwidth of a cross polarization antenna is about 90°. However, the desired sector beamwidth is typically about 65° for a three-sector base station configured with a cloverleaf cell layout. Although a single antenna with a 65° beamwidth is commercially available, per antenna beamwidth this small is difficult to achieve in a closely spaced antenna array. To achieve the desired sector beamwidth for the common broadcast channels using a closely spaced antenna array, it is necessary to use two or more antennas to narrow the common broadcast channels by beamforming. This also helps to share the transmit power of the common channels among power amplifiers. In summary, there are two types of beamforming for transmission from a base station to a mobile: (1) an individual narrow beam that contains traffic and mobile specific information steered towards a target mobile, (2) a broader sector-wide fixed beam that contains common broadcast channels pointed towards the central line of the sector to cover all mobiles under the sector coverage.
In order to form a beam using closely spaced multiple antennas, calibration is required to equalize the amplitudes and phases among all antenna branches in the path between the radio and antenna. Angle of departure from the antenna array is determined by the phase difference between digital signals coming out of the radios. Calibration is usually performed digitally in the radios by using calibration signals to measure the amplitude and phase differences in the appropriate paths. The calibration coefficients or weights are then applied to the receive or transmit signals to compensate for the path differences. In the more common Frequency Division Duplex (FDD) system, the carrier frequencies in the forward and reverse links are different. Hence separate calibrations must be performed in forward link and reverse link at different frequencies. However, in a Time Division Duplex (TDD) system, the forward link and reverse link share the same carrier frequency by occupying different time slots. Ideally, by reciprocity, the forward link gain at the antenna and reverse link gain at the radio receiver are reciprocal of each other. If the amplitude and phase of the signal received from a mobile in the reverse link are estimated by the radio, then the amplitude and phase are equal to the reciprocal of the received signal.
In a practical implementation, the phase response of an amplifier can vary from unit to unit and the variation may also be frequency dependent. Since separate amplifiers are used for the receive (low noise amplifier) and transmit (power amplifier) paths, reciprocity does not hold for all the branches. Therefore, it is necessary to calibrate the transmit/receive amplifier loops among all the branches. This calibration enables proper beamsteering for the traffic signals for mobiles. Reciprocity still holds from beyond the amplifiers to the antennas, which consists mainly of the RF cables between the ground equipment to the antennas at the top of the tower. Hence calibration for this RF cable portion is not required for the traffic signals in a TDD system. However, calibration of the RF cable portion is required if fixed beamforming is required for transmitting common broadcast channels. Angle of departure is usually at boresight or 0°.
One calibration technique that has been proposed for an OFDM TDD system uses one mobile unit antenna to measure the phase and amplitude or complex gain of the signal transmitted from each of the antennas in the antenna array at the base station. For example, the mobile unit can measure the complex gain of a forward link pilot channel transmitted by the base station over the air. The forward link pilot channel may use different sub-carriers to identify the branch that transmits the pilot so that the signals from the antennas can be separated by the mobile unit. The mobile unit feeds back the complex gains of all branches to the base station. In the meantime, the base station measures the received complex gains through all the base station antennas for reverse link signals. A set of complex calibration coefficients is computed by taking the ratio of the forward link gain to the reverse link gain. This set of calibration coefficients is independent of mobile location in the sector due to reciprocity of TDD because the over-the-air portions of channel propagation for the forward and reverse links are the same and (theoretically) they cancel out in computing the ratio. The final forward link beamsteering weights for each mobile unit are computed by multiplying the calibration coefficients with the corresponding reverse link complex gain for the target mobile. Since the calibration coefficients are not dependent on the mobile location, the coefficients derived from one mobile unit can be applied to other mobile units at different locations in the same sector.
This mobile-assisted calibration method has three primary drawbacks. First, although gain values from only one mobile unit in good RF condition are required, all mobile units must be equipped with the feedback feature so that they are capable of sending back the gain information to the base station. Information received from more than one mobile unit is redundant. Second, this calibration method does not calibrate the RF cables to allow the base station to transmit fixed beamwidth common broadcast channels in boresight; the beamwidth is usually 65° in a three-sector system. Third, phase and amplitude among amplifiers may not track each other over a wide bandwidth, e.g. 20 MHz. Consequently, the bandwidth needs to be divided into small sub-bands and each sub-band has to be calibrated over a smaller bandwidth.
One alternative calibration method relies on the coupling between closely spaced antennas. In this method, a calibration signal is transmitted from one antenna while the other antennas in the array receive coupled signals corresponding to the transmitted calibration signal. The coupled signals are received and processed by the corresponding radio receivers. The calibration coefficients are derived from the phase and amplitude of each coupled signal minus the corresponding coupling factor at the antennas. The calibration path therefore includes all the cables and RF components, but does not include the antenna coupling. This method has two drawbacks. First, the coupling factor between a pair of antennas must be known within a certain range of tolerance. Unfortunately, the coupling factors are not usually reliably known. For example, the coupling value may vary widely from unit to unit due to high sensitivity to near field conditions and manufacturing precision, particularly at higher carrier frequencies. Furthermore, the coupling value can be changed by degradation over time and weather conditions, and/or other effects. Variations in the coupling value may compromise the accuracy of calibration. Matching of coupling among antenna array units is necessary in the manufacturing process. Second, one of the antennas must be in transmit mode while the other antennas are in receive mode. Consequently, beamsteering will be interrupted during the calibration process.